<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    
</body>
<script>
    let num1 ,num2 = 0,num3 = 123,str1 = '',str2 = 'hello', flag=null;
    console.log(Boolean(num1)); //未定义型转布尔型：false
    console.log(Boolean(num2)); //数值0转布尔型:false
    console.log(Boolean(num3)); //非0数值转布尔型：true
    console.log(Boolean(str1)); //空串转布尔型：false
    console.log(Boolean(str2)); //非空字符转布尔型:true
    console.log(Boolean(flag)); //null转布尔型:false
   // Boolean()函数会将任何非空字符串和非零的数值转换为ture,
    //将空字符串，0，NaN,undefined和null转换为false.
    console.log('---------');
    //未定义转数值
    console.log(Number(num1));
    console.log(parseInt(num1));
    console.log(parseFloat(num1));
    console.log('---字符转数值-----');
    //字符型转数值
    console.log(Number(str1)); //空串转数值：
    console.log(parseInt(str1)); //空串转数值：
    console.log(parseFloat(str1)); //空串转数值：
    console.log(Number(str2));  //非空串转数值：
    console.log(parseInt(str2)); //非空串转数值：
    console.log(parseFloat(str2));//非空串转数值：
    console.log('---------');
    let str4 = 'hello4.34';
    let isNaN;
    isNaN = Number(str4);
    console.log(isNaN);  //NaN
    console.log(Boolean(isNaN)); //false
    
</script>
</html>